It depends. InvisionFree is free.
Although, InvisionPowerBoard is not free. It is quite expencive. Same with vBulletin. That is also expencive.
If you want a free forum, and you have your own webspace, then try phpBB. Or, if you have no webspace, InvisionFree is the way to go. Although, Aimoo is also good.. but the ads can get annoying.